Traditionally it was thought that palatability, through its action on reward centers in the brain, played a major role in reinforcing feeding. However, it has become increasingly clear that calories/nutrients, not palatability, act as primary post-ingestive reinforces on feeding. Animals can determine the nutritional value of sugar without the influence of taste.
Hunger is a powerful drive that stimulates food intake. The means by which hunger influences the brain to promote feeding behavior is unclear, however. The sensation of hunger is a complex motivational state that promotes multiple facets of feeding behavior. Food deprivation increases subsequent food consumption and other behaviors related to food seeking that results in the selection of nutrient-rich food and enhances sensory sensitivity in both insects and vertebrates. The mechanism by which energy deficits are represented in the brain and incorporated to modulate feeding behaviors is not well understood in any animal system. Drosophila provides a model for studying the neural correlates of hunger. Its genetically amenable system affords the unbiased screening of cells or populations of neurons in the adult brain that are crucial for mediating the motivational states induced by hunger.